The invention concerns a linear rolling bearing for transmission of torques about a central longitudinal axis, comprising an inner profiled element and an outer profiled element at least partly surrounding the inner profiled element, wherein
the inner and outer profiled elements are arranged for sliding relative to each other in longitudinal direction,
the inner profiled element has a non-circular cross-section and at least two torque-transmitting, outwards oriented outer wall sections,
at least one torque-transmitting inner wall section of the outer profiled element is associated to each outer wall section,
the outer wall sections and the inner wall sections are mounted on each other by rolling elements,
the rolling elements are guided in at least one rolling element circuit and
the rolling element circuit guides the rolling elements in longitudinal direction of the linear rolling bearing in a first guide track through a loaded zone and thereafter, in a second guide track through a load-free zone.
Linear rolling bearings are used in almost all fields of mechanical and automobile engineering. Such bearings serve to mount longitudinally displaceable sliding parts on each other. When used with variable-length telescopic shafts, a bearing of this type must additionally transmit torques transmitted to it from the shaft. Variable-length telescopic shafts are used, for example, as steering shafts in steering columns of modern automobiles. Such steering columns permit the adjustment of the steering wheel position in the interior of the vehicle to suit the individual height and body position of the vehicle operator. This changes the absolute distance of the steering wheel from the steering gear. This change of distance can be corrected by two telescopically slidable shaft ends. Since the steering shaft transmits the steering torque from the steering wheel to the steering gear, the steering shafts have also to be rotationally coupled with each other. The position of the steering wheel is adjusted either manually by the body force of the operator or with the help of electromotors, in both cases, the forces occurring in the sliding fit of the telescopic shafts should not be too high. With the help of a linear rolling bearing disposed between the two shaft ends, the sliding forces are held at a low and relatively constant level over the entire sliding range. Occasionally, steering shafts are also subjected to length variations during vehicle operation. These are short-stroke, oscillating length variations and are caused by a relative movement between the steering gear, together with the lower steering shaft end, relative to the fixed upper steering shaft end.
The linear rolling bearings used are of different types. Frequently, for reasons of economy, linear bearings used in the aforesaid case permit only a limited stroke of one component relative to the other. As a rule, these bearings comprise cage-guided rolling elements situated between the two relatively slidable parts. The cage has a tendency to migrate during operation, particularly in the presence of short-stroke movements. This results in a sliding motion of the rolling elements rather than a rolling motion. In this way, the cage reaches a position in which the operational stroke of the cage is no longer guaranteed. As a result, when larger strokes occur, but also upon short-stroke movements, the rolling elements and the cage are subject to forced movements which result in sliding and impermissible wear.
At first sight therefore, one would consider using a prior art linear rolling bearing that permits an unlimited sliding motion. Such bearings comprise one or more ball circuits in which the balls circulate successively through a loaded zone and a load-free return zone and then again through the loaded zone. In a linear rolling bearing of this type known from DE 33 08 831 A1, a bushing comprises inner and outer rolling bearing raceways and is surrounded on the inside and the outside by cage halves. The cage-guided rolling elements roll from the loaded inner zone on the inner diameter of the bushing and the outer contour of the sliding shaft to and over the outer contour of the bushing and from there, again into the loaded zone. With such bearings, the drawbacks of the aforementioned linear rolling bearings for a limited stroke are avoided. But they have a relatively complicated structure and are not free of play.
For reasons of comfort and smooth operation, a play-free construction of such bearings is often required. Play in the mounting between two ends of a steering shaft means a deterioration of the quality of steering and lack of comfort as a result of possible rattling noises. The quality of steering deteriorates because radial play between the two steering shaft ends manifests itself as a rotary play in the steering wheel. The rattling noises mentioned above are perceived acoustically or through vibrations at the steering wheel and have an unpleasant effect on the operator. The initially mentioned linear rolling bearings with a limited stroke, in contrast, can be configured without play with relatively simple means. For example, EP 0 251 723 B1 describes a torque-transmitting linear rolling bearing in which rolling bearing raceways are supported on the inner component by spring elements. The spring elements urge the rolling elements against the outer component so that a play-free seating of the components relative to each other is assured. Due to the rolling motion of the rolling elements, the displacement of the components relative to each other in longitudinal direction is rendered smoother and is accompanied by low wear only. The components are arranged relative to each other with a very small gap having a dimension which is xe2x89xa6 the spring excursion of the spring elements. The spring elements are designed so that in the presence of normal torsional forces, the rolling elements can absorb any torque. If, however, one component is rotated relative to the other under the action of an excessive torque, the springs deflect to the extent that the components come to bear against each other. The torque is then transmitted directly through the walls of the components, and the rolling elements are relieved of load. The spring elements must be adapted to normal torsional forces. Thus, they function with a relatively high pre-stress even in normal operation. This high pro-stress is detrimental to the running smoothness required under certain circumstances of a linear bearing.
It is an object of the invention to create a linear rolling bearing which avoids the drawbacks of the aforementioned types of linear rolling bearings.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves these objects by creating a linear rolling bearing in which the rolling element circuit is arranged in a cage and the cage is arranged between an inner wall section of an outer profiled element and an outer wall section of an inner profiled element, one of the inner and the outer profiled elements comprising at least one biasing means acting on the rolling elements in the loaded zone of the rolling element circuit to create a play-free biasing of the inner and outer profiled elements against each other. Such a linear rolling bearing is simple and economic to manufacture. Both the inner and the outer profiled elements are simple to configure and do not need to be provided with additional rolling element raceways, retainers for the cage or other elements for the rolling element circuits. The rolling element circuit is configured-in the rolling element cage. The guide tracks of the rolling elements which are connected to each other to form a circuit, are made exclusively in the cage. The free space required for the return of the unloaded rolling elements can be created with simple means, e.g. in the form of a groove having relatively large tolerances that can be made economically in one or both of the profiled elements. The advantages of a linear rolling bearing with rolling element circuits are utilized while the initially mentioned drawbacks are eliminated.
It is very simple to render such a bearing free of play. This is accomplished by the fact that the rolling elements situated in the loaded zone are urged by a biasing means against a wall section of one or both of the profiled elements, while the biasing means itself is arranged on one or both of the profiled elements. One possibility is to use spring sheets configured as raceways. The spring sheets can be made in a simple manner by punching and bending or stamping out of spring steel or a similar hardened material and have, for example, an arched cross-sectional profile. The spring sheet is arched, as viewed in cross-section, by a dimension that is at least minimally larger than the largest possible theoretical play that can result from manufacturing tolerances in the bearing. Pre-stressed in the mounted state in the linear rolling bearing, the Spring sheet produces a biasing force on the loaded rolling elements which run thereon so that the loaded rolling elements are urged against the opposite wall section. Such spring sheets can also be inserted into both the opposing or associated wall sections. The sheet thickness of the spring sheets and/or geometry of arching is chosen as a function of the required pre-stress. But the pre-stress does not have to be that large for the transmission of torques through the spring shout and the rolling elements to the other wall section. It is sufficient if these spring sheets are designed to produce a pre-stress level that renders the linear bearing free of play for longitudinal movements and in operation.
In one embodiment of the invention, the biasing means is constituted by at least one raceway section of elastic configuration situated in the loaded zone and formed on at least one of the wall sections associated to each other. The already mentioned arched spring sheet is advantageously used in this embodiment as well. In this case, the spring sheet is arched, as seen in cross-section, only in the section in which the loaded zone is situated. The spring action is the same as described in the foregoing paragraph. Alternatively, it is possible to use a substantially flat sheet which is supported on one of the wall sections and produces the pre-stress on the loaded rolling elements by leverage. Such spring sheets can be produced at a minimum cost. The spring pre-stress can be influenced by the sheet thickness and the lever length.
In another embodiment of the invention, when torques are introduced, the loaded zone defects to the extent that the associated wall sections approach each other till at least one section of the cage transmits the torques from one of the associated wall sections to the other of the associated wall sections. The cage is thus not only a means for guiding the rolling elements but also a means for the transmission of torques. The cage is arranged at a relatively small distance from each of the walls. The distance of the cage from the walls determines the magnitude of the torque which is initially transmitted only by the rolling elements. When this torque is exceeded, the loaded zone deflects and the walls approach each other till the cage, or a section of the cage, is clamped therebetween. The magnitude of the torque that is still transmitted by the rolling elements and the raceways can be defined by the configuration of the elastic elements in the loaded zone and, as already mentioned, by the distance of the cage from the walls. It is therefore proposed in a further embodiment of the invention that the raceway section situated in the loaded zone have an elastic configuration. Such a raceway section can be formed on one wall section, either of the inner or the outer profiled element, or on both the wall sections associated to each other. It is further proposed in this embodiment of the invention that upon introduction of torques, the elastic raceway sections deflect to the extent that the associated wall sections approach each other till a section of the cage is clamped therebetween or till they come into direct contact with each other.
In further embodiments of the invention, the inner profiled element is formed by an inner ring of the linear rolling bearing and the outer profiled element is formed by a bushing of the linear rolling bearing. Such elements can be manufactured economically, especially in mass production, by chipless shaping but also by machining. The advantage of the production of individual elements is that the tubes or shafts in or on which such a linear rolling bearing is used can have a relatively simple configuration. Preferably, the inner or outer ring is pressed into or onto the simply configured profiled elements. However, it is also conceivable that the inner wall sections or the raceways for the rolling elements in the loaded zone be defined directly by the surface of the telescopically slidable components. In this case, it is particularly advantageous if the shaft elements and the associated outer tubes have a simple geometry. A further embodiment of the invention therefore provides that the inner profiled element has a polygonal cross-section with at least three outer wall sections. Three outwards oriented wall sections assure that the forces resulting from torque introduction are uniformly distributed on all the loaded zones of the linear rolling bearing. Irrespective thereof, rectangular, cuboid or other polygonal cross-sections may also be used.
As already mentioned initially, parts made of spring sheet can be configured and manufactured simply and economically. In a further embodiment of the invention therefore, a thin-walled, elastic raceway is provided on one or both of the associated wall sections, and, in the region of the load-free zone, this raceway comprises a longitudinal groove. Such a raceway which is made preferably by stamping, assures that the rolling elements are relieved of load and guided in their circuit. In this embodiment, too, the raceway deflects, at least in the loaded zone, to the extent that the associated wall sections approach each other till they come into contact or till a section of the cage is clamped therebetween. Sufficient free space is created in the respective profiled element for this deflection. The spring sheets are inserted and snapped into the profiled elements in a simple manner and fixed by welding or by other means.
Finally, the invention also provides that the inner profiled element is formed by a section of a steering shaft and the outer profiled element is formed by a second section of a steering shaft of a steering of an automotive vehicle. The majority of components used in steering columns of modern automotive vehicles are made by chipless shaping out of sheet metal. Such steering columns must be simple to manufacture and must have a low weight, a small design space requirement and must function reliably and comfortably. For this reason, the linear bearing of the invention with all its embodiments is particularly suitable for use between telescopically slidable steering shafts.
The invention will now be described with the help of several examples of embodiment illustrated in the appended drawings.